Triton: What Origin for this Unusual Moon?

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TritonWhat Origin forthis Unusual Moon?

• Jared Leisner
• November 18, 2004

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Introduction

• Summary of the Neptune planetary system
• Satellites
• Rings
• Possible origins of Triton
• Consequences of each origin
• Needed events to evolve from each origin to the
  current system

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Neptune Planetary System

• Satellites
• Inner satellites
• Naiad, Thalassa, Despina, Galatea, and Larissa
• Triton
• Nereid
• Rings
• Arcs
• Not continuous rings circling the planet

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Neptune System Inner Satellites
Modified from Banfield and Murray (1992)
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Neptune System Inner Satellites

• Orbital parameters
• Eccentricity Close to circular
• Inclination Close to zero
• Except for innermost satellite, about 4.7
• Semi-major Axis Confined to five Neptune radii
• Prograde
• Physical parameters
• Diameter 60 to 200 km, linearly increasing
  outward
• Irregularly shaped, no sign of geological
  modification

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Neptune System Triton

• Orbital parameters
• Eccentricity Close to circular
• Inclination 157.3
• Semi-major Axis 14.4 Neptune radii
• Retrograde

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Neptune System Triton, con't

• Physical characteristics
• Similar to Pluto
• Density 2.1 g cm-3 (high rock-to-ice ratio)
• Diameter 2705 km
• Mass 2.14E22 kg
• Five hundred times the mass of the other moons
  combined
• High crater asymmetry with low crater count
• Craters on 30 of the surface, concentrated on
  the leading face

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Neptune System Nereid

• Orbital parameters
• Eccentricity 0.7152
• Inclination 27.6
• Semi-major axis 223.9 Neptune radii
• Prograde
• Physical parameters
• Diameter 340 km
• Not much else is known

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Neptune's Rings

• Instead of continuous rings like those that
  encircle other planets, Neptune's rings are
  broken into discrete arcs, pointed to in yellow.
• They appear to be close, but not in, a resonance
  with the inner satellite Galatea, pointed to in
  white (Sicardy et al., 1999).

From Sicardy et al. (1999)
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Origin 1 Formed withinNeptune's Planetary
System

• Perhaps Triton was created out of the original
  planetary nebula
• Consequences for Triton
• Orbital direction (prograde)
• Inclination (nominal)
• Surface activity (similar to others?)
• Consequences for planetary system
• Distribution, characteristics of satellites
  (nominal)
• Full rings instead of just arcs?

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Origin 1 To the Present

• How did Triton go from a prograde orbit to a
  retrograde orbit?
• A collision with a passing body
• Would require an Earth-sized planetsimal
• Neptune was originally retrograde and switched
• Nominal eccentricity and inclination do not
  suggest such a radical change
• Why would the rest of the satellites be
  now-prograde?
• It formed retrograde
• How?!
• Why the atypical distribution of satellites and
  their orbital parameters?

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Origin 2 Captured via TidalFriction or
Third-Body Interactions

• Over a reasonable timescale, tidal effects can
  not dissipate enough orbital energy for Triton to
  be captured unless periapsis was extremely close
  to Neptune (McKinnon and Leith, 1995).
• This would have left Triton in a more tightly
  bound orbit than is now observed.
• Solar tides make capture more difficult by
  oscillating the satellite's orbit and a
  Pluto-assisted capture is accepted as impossible
  (McKinnon and Leith, 1995).

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Origin 3 Captured via Gas Drag

• McKinnon and Leith (1995) modelled a gas drag
  capture of Triton while there was a significant
  Neptune nebula (resembling that modelled for
  Uranus) still in existance.
• This model puts Triton close to its present day
  situation in 103-5 years, depending on solar
  tides.
• Triton's eccentricity would have been left at
  0.2, which would have left it open to
  non-trivial tidal heating.

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Origin 3 Problems withGas Drag Capture

• Gas drag acts to decrease the inclination of a
  body's orbit and makes said more prograde.
• These effects, coupled with the observed orbit of
  today, place upper bounds upon the amount of gas
  drag that may have worked upon Triton. Those
  upper bounds then imply lower bounds upon the
  amount of orbital evolution due to tidal heating.

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Origin 4 Capture by Impact

• Before Voyager 2 reached Neptune, Goldreich et
  al. (1989) modelled the capture of Triton as it
  entered the Neptune system and struck a natural
  satellite with a mass a few percent of its own.
• Using a somewhat crude argument of gravitational
  focusing, the authors calculated that more than
  104 bodies of Triton's size may have passed
  within 10 Neptune radii which, if Neptune
  originally had a system akin to Uranus', would
  yield a chance for this Triton collision-capture
  of several tens of percent.
• Goldreich et al. calculated that if Triton (with
  a k20.1 and Q100) entered an elliptical orbit
  with a periapsis of 7 Neptune radii and semimajor
  axis of 103 Neptune radii, then Triton could
  evolve, through tidal dissipation, to its present
  situation in less than one billion years.

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Origin 4 Consequences

• It followed from the model proposed by Goldreich
  et al. (1989) that Neptune would be devoid of
  satellites between 5 Neptune radii and Triton's
  current orbit.
• When Voyager arrived three months later, this was
  found to be precisely the case.
• The calculations also correctly lead to Nereid's
  irregular orbit as Triton cross the former's
  orbit 108 times, perturbing its semi-major axis,
  eccentricity, and inclination a few tenths of a
  percent each time.
• A last implication of this model would be an
  inner satellite shepherding the ring arcs in a
  slightly inclined orbital path.
• While Galatea does appear to shepherd the ring
  arcs, it is not the one inclined.

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Geologic Activity of Triton

• The low count of impact craters implies a
  relatively young age for Triton's surface. As low
  as 100 Myr, and it is possible that it is still
  active (Stern and McKinnon, 1999 Ruiz, 2003).
• This lower bound for the age, if accurate,
  implies that the moon underwent significant
  heating this heating would be easy to explain
  with tidal dissipation as its orbit around
  Neptune evolved.

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Recently Discovered Moons

• From Holman et al. (2004)
• Centered on Neptune. The red (blue) circle
  indicates stability of prograde (retrograde)
  satellites and the color of satellite's label
  indicates it's orbit.
• c02N4 was lost.
• Presumably captured satellites, by their orbits.

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Conclusion

• Triton's origin? A collision capture.
• The predictions for Goldreich et al.'s (1989)
  model, save the inclined shepherd, were shown by
  Voyager's subsequent flyby to be true.
• The orbital evolution after that capture would
  lead to geologic activity and the young surface
  that is observed.
• The detection of five new irregular moons, albeit
  of significantly smaller proportions than Triton,
  that would appear to be captured satellites lends
  credence to the idea that this would not be
  impossible.

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References

• Banfield, D. and N. Murray, 1992. A dynamical
  history of the inner Neptunian satellites. Icarus
  99, 390-401.
• Goldreich, P., N. Murray, P.Y. Longaretti, and D.
  Banfield, 1989. Neptune's story. Science 245,
  500-504.
• Holman, M.J., J.J. Kavelaars, T. Grav, B.J.
  Gladman, W.C. Fraser, D. Milisavljevic, P.D.
  Nicholson, J.A. Burns, V. Carruba, J. Petit, P.
  Rousselot, O. Mousis, B.G. Marsden, and R.A.
  Jacobson, 2004. Discovery of five irregular moons
  of Neptune. Nature 430, 865-867.
• Marachi, S., C. Barbieri, and M. Lazzarin, 2004.
  Mass transfer in the satellite system of Neptune
  implications for Triton's crater asymmetry.
  Planetary and Space Science 52, 671-677.
• McKinnon, W.B. And A.C. Leith, 1995. Gas drag and
  the orbital evolution of a captured Triton.
  Icarus 118, 392-413.
• Ruiz, Javier, 2003. Heat flow and depth to a
  possible internal ocean. Icarus 166, 436-439.
• Stern, S.A. and W.B. McKinnon, 2000. Triton's
  surface age and impactor population revisited in
  light of Kuiper Belt fluxes evidence for small
  Kuiper Belt Objects and recent geologic activity.
  The Astronomical Journal 119, 945-952.